Fossil fueled steam boilers are commonly used throughout the world for steam generation for electrical power companies, for pulp wood and paper mills, etc. These are typically high pressure, high temperature, high capacity boilers often well over 100 feet in height and adaptable to a variety of fuels including pulverized coal, natural gas and oil and even paper mill waste slurries. In order to facilitate steam generation, most modern boiler furnaces have inner walls consisting of an array of tubes or passages through which water is circulated.
Boilers are constructed in a wide variety of sizes and shapes (which are largely determined by the power requirements and the type of fuel to be used). While most boilers are typically rectangular in plan, the height may vary considerably. The bottom of the boiler may be flat, slightly sloped, or more commonly of "V" shape with the bottom of the "V" open to allow for coal ash to fall into an ash trough.
In the case of coal burning systems and, to a lesser extent with oil, a very important operational consideration is the presence of ash and noncombustible particulates in the fuel. During operation of the boiler, these byproducts of combustion accumulate on the boiler walls and floors to create a deleterious insulating condition. This accumulation not only reduces the amount of heat absorbed by the water jacket, but also increases draft loss, reduces efficiency, corrodes pressure parts, and eventually can force shutdown of the boiler unit for cleaning, maintenance and repairs. The sulfur containing residues in the paper mill boilers can be particularly noxious and corrosive, causing piping blowouts.
A formal inspection and maintenance program is essential to the continuous safe and efficient operation of any power generating boiler. Prior to the initiation of any maintenance and repair procedure an interior scaffolding latticework must be erected within the boiler combustion chamber to provide the necessary platform for a number of workmen simultaneously and efficiently to clean and repair the inside boiler wall. The installation of the necessary scaffolding represents a considerable expense both in labor and material as well as lost revenues due to the down time of the boilers and the associated steam turbines. It is extremely important in such an installation to keep boiler outages and downtime to a minimum. Reportedly, the cost to a power utility to shut down a boiler can run as high as several hundred thousand dollars per day depending on power costs and power demands.
The type of scaffolding which can be used is limited, because access openings to the boiler interiors are normally very limited in size and number. Often there is only one manhole opening available which typically ranges in diameter only from 18 to 30 inches. The throat opening to the ash pit may be 21/2 feet wide, but often must be planked over to protect maintenance workers in the ash pit (and is therefore not available as an access). Furthermore, access to the ash pit itself can be similarly limited. Normal scaffolding frames would not fit and thus cannot be used for such jobs.
Early methods employed suspended scaffolding (which was slow and difficult to use). Only a few workers could be accommodated at a given time. Custom built wooden scaffolding and then tube-and-coupler scaffolding were apparently also used at one time. Although accommodating of more workers, these also were too slow and costly. Later, a standing latticework of modularized post-and-runner type scaffolding began to be used. To erect and support such scaffolding, one had to weld fixed support members at precise, previously determined, locations along the sloped converging bottom walls of the boiler. The scaffolding latticework was then installed inside the boiler with support being provided by vertical posts which were either mounted to the welded support on the sloping bottom walls of the boiler or to a truss spanning the throat. In this method, the entire weight of the scaffolding was transmitted directly to and supported by such bottom walls through these weldments. These approaches created several problems both in installation and removal. The positioning and welding of the support members along the lower converging boiler walls had to be precisely arranged in order to properly support the modularized scaffolding system. Both of these requirements necessitated a labor force with higher skill levels that further increased labor costs. Also, the physical welding of each of the support members was labor intensive and time consuming. Once maintenance, inspection and/or repair work had been completed, the scaffolding and its welded support brackets had to be removed. If left in the boiler, the brackets could not be relied upon to survive intact, because of the hostile environment within the boiler when the firebox was in use. This breaking and removal of the weldments was a further cost and created another potential for damage to the boiler walls.
One method which has been used to eliminate the the need for the welded scaffolding support bracket on the V-shaped bottom walls is shown in U.S. Pat. No. 4,496,026. This shows a founding ladder system which provides support for a scaffolding system mounted inside a boiler. This founding ladder system (like others before it) incorporates a truss mounted horizontally in the boiler throat. Connected to the truss are ladder-like supports which extend up and rest on the inside convergingly sloped bottom walls of the boiler. These ladder-like supports through adjustable attachments provide movable bases along the sloping walls from which to erect the scaffolding system to be constructed inside the boiler. These ladder-supports also serve to redirect much of the weight from scaffolding posts down to the horizontal throat trusses, to which they are connected. This system, however, requires a large inventory of relatively specialized complex members which are expensive to fabricate, inventory and maintain. Being specialized, they have little use between boiler jobs. Also, this support system utilizing the founding ladders requires extensive modification where obstructions inherent in the converging boiler bottom walls interfere with the normal symmetric placement of the founding ladder supports.